[LNT] Switch docker image to Python 3

Summary:
With Python 2 end of support being so near, this commit changes the
docker image to be in Python 3 rather than Python 2.

Reviewers: cmatthews, hubert.reinterpretcast, kristof.beyls, leandron, PrzemekWirkus

Reviewed By: PrzemekWirkus

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D69053

Author: Thomas Preud'homme

docker/Dockerfile

@@ -1,10 +1,10 @@
-FROM python:2.7-alpine
+FROM python:alpine
 
 RUN apk update \
   && apk add --no-cache --virtual .build-deps git g++ postgresql-dev yaml-dev \
   && apk add --no-cache libpq \
   && git clone https://git.llvm.org/git/lnt /var/src/lnt \
-  && python /var/src/lnt/setup.py install --server \
+  && python3 /var/src/lnt/setup.py install --server \
   && rm -rf /var/src \
   && apk --purge del .build-deps \
   && mkdir /var/log/lnt